This 3-generation study of families at high and low risk for Major Depressive Disorder (MDD) has documented the strong familial transmission of mood disorders across generations. We have conducted 5 waves of assessments in this cohort over 25 years. In the 5th wave, we added Magnetic Resonance Imaging (MRI) measures and found a remarkably robust association of familial risk for MDD with asymmetries in cortical thickness, with a nearly 30% reduction in thickness observed in the lateral parietal, temporal, and frontal cortices of the right hemisphere of the high risk group. These MRI findings are consistent with EEG findings from the 4th wave of assessments that demonstrated reduced activity over the posterior cortices of the right cerebral hemisphere. Both the MRI and EEG findings were present in high-risk individuals who never had MDD in their lifetimes, suggesting that these abnormalities were not simply a consequence of previously having been depressed or having been treated for depression. Thinning of the cortical mantle and reduced electrophysiological activity in the right hemisphere therefore may constitute related endophenotypes for familial vulnerability to developing MDD. We are proposing a 6th wave of study to gain a deeper understanding of the right hemisphere abnormalities in familial MDD in the 216 individuals imaged thus far. This represents the largest MRI study published for MDD to date, and it is the only sample studying 3 generations of individuals at high or low risk for MDD. We are proposing to collect additional MRI and EEG measures, as well as clinical and cognitive neuroscience data, that will inform us about the neural bases of the right hemisphere thinning and their consequences for brain function and emotional processing. We will also determine whether additional cortical thinning in the left cerebral hemisphere predicts new or recurrent MDD in those people who were imaged in Wave 5. Our cohort has exceptional attributes that will aid our search for detecting MDD endophenotypes, including (1) an elevated clinical risk for developing MDD in the high risk group that has been amply demonstrated;(2) multiple prospective assessments, conducted blind to risk status, provide great confidence in the accuracy of the clinical diagnoses;and (3) the sample contains individuals who have not yet become ill but who are nevertheless at elevated risk of becoming ill, thereby allowing us to disentangle the neurobiological determinants of vulnerability from the compensatory responses that would be present in already-affected individuals. MDD is an early-onset, highly prevalent disorder with significant morbidity, and it afflicts people in their most productive years. Identifying the biological vulnerability for depression before the onset of illness has important implications for prevention and public health. PUBLIC HEALTH RELEVANCE: MDD is an early-onset, highly prevalent disorder with significant morbidity, and it afflicts people in their most productive years. Identifying the biological vulnerability for depression before the onset of illness, as we have described in the study, has important implications for prevention and public health.